Expression of viral CD45 ligand E3/49K on porcine cells reduces human anti-pig immune responses

Transgenic expression of protective molecules in porcine cells and tissues is a promising approach to prevent xenograft rejection. Viruses have developed various strategies to escape the host's immune system. We generated porcine B cells (B cell line L23) expressing the human adenovirus protein E3/49K or the human cytomegalovirus protein pUL11 and investigated how human T, NK and B cell responses are affected by the expression of the viral proteins. Binding studies revealed that E3/49K and pUL11 interact with CD45 on human but not porcine peripheral blood mononuclear cells. T cell proliferation in response to L23-E3/49K cells was significantly reduced and accompanied by development of an anti-inflammatory cytokine milieu (low: TNF-alpha, IFN-gamma, IL-6; high: IL-4, IL-10). Human peripheral blood mononuclear cells which had been primed for four weeks by L23-E3/49K cells included an extended population of regulatory T cells. Cytotoxicity of effector T and natural killer cells against L23 cells was significantly reduced (40 to 50%) by E3/49K expression. B cell activation and antibody production to E3/49K expressing cells was also diminished. Surprisingly, pUL11 expression showed no effects. Reduction of human anti-pig immune responses by transgenic expression of selected viral genes may be a novel approach for protection of porcine xenografts.

Triple (GGTA1, CMAH, B2M) modified pigs expressing an SLA class Ilow phenotype-Effects on immune status and susceptibility to human immune responses

Porcine xenografts lacking swine leukocyte antigen (SLA) class I are thought to be protected from human T cell responses. We have previously shown that SLA class I deficiency can be achieved in pigs by CRISPR/Cas9-mediated deletion of β₂ -microglobulin (B2M). Here, we characterized another line of genetically modified pigs in which targeting of the B2M locus did not result in complete absence of B2M and SLA class I but rather in significantly reduced expression levels of both molecules. Residual SLA class I was functionally inert, because no proper differentiation of the CD8⁺ T cell subset was observed in B2M^low pigs. Cells from B2M^low pigs were less capable in triggering proliferation of human peripheral blood mononuclear cells in vitro, which was mainly due to the nonresponsiveness of CD8⁺ T cells. Nevertheless, cytotoxic effector cells developing from unaffected cell populations (eg, CD4⁺ T cells, natural killer cells) lysed targets from both SLA class I⁺ wildtype and SLA class I^low pigs with similar efficiency. These data indicate that the absence of SLA class I is an effective approach to prevent the activation of human CD8⁺ T cells during the induction phase of an anti-xenograft response. However, cytotoxic activity of cells during the effector phase cannot be controlled by this approach.

Possible detrimental effects of beta-2-microglobulin knockout in pigs

BACKGROUND

Despite major improvements in pig-to-primate xenotransplantation, long-term survival of xenografts is still challenging. The major histocompatibility complex (MHC) class I, which is crucial in cellular immune response, is an important xenoantigen. Abrogating MHC class I expression on xenografts might be beneficial for extending graft survival beyond current limits.

METHODS

In this study, we employed the CRISPR/Cas9 system to target exon 2 of the porcine beta-2-microglobulin (B2M) gene to abrogate SLA class I expression on porcine cells. B2M-KO cells served as donor cells for somatic cell nuclear transfer, and cloned embryos were transferred to three recipient sows. The offspring were genotyped for mutations at the B2M locus, and blood samples were analyzed via flow cytometry for the absence of SLA class I molecules.

RESULTS

Pregnancies were successfully established and led to the birth of seven viable piglets. Genomic sequencing proved that all piglets carried biallelic modifications at the B2M locus leading to a frameshift, a premature stop codon, and ultimately a functional knockout. However, survival times of these animals did not exceed 4 weeks due to unexpected disease processes.

CONCLUSION

Here, we demonstrate the feasibility of generating SLA class I knockout pigs by targeting the porcine beta-2-microglobulin gene using the CRISPR/Cas9 system. Additionally, our findings indicate for the first time that this genetic modification might have a negative impact on the viability of the animals. These issues need to be solved to unveil the real value for xenotransplantation in the future.

Strong xenoprotective function by single-copy transgenes placed sequentially at a permissive locus

BACKGROUND

Multiple xenoprotective transgenes are best grouped at a single locus to avoid segregation during breeding and simplify production of donor animals.

METHODS

We used transgene stacking to place a human CD55 transgene adjacent to a human heme oxygenase 1 construct at the porcine ROSA26 locus. A transgenic pig was analyzed by PCR, RT-PCR, droplet digital PCR, immunohistochemistry, immunofluorescence, and flow cytometry. Resistance to complement-mediated cell lysis and caspase 3/7 activation were determined in vitro.

RESULTS

The ROSA26 locus was retargeted efficiently, and animals were generated by nuclear transfer. RNA and protein analyses revealed abundant expression in all organs analyzed, including pancreatic beta cells. Transgenic porcine kidney fibroblasts were almost completely protected against complement-mediated lysis and showed reduced caspase 3/7 activation.

CONCLUSION

Step-by-step placement enables highly expressed single-copy xenoprotective transgenes to be grouped at porcine ROSA26.

Inhibition of B-cell activation and antibody production by triggering inhibitory signals via the PD-1/PD-ligand pathway

BACKGROUND

The development of donor-reactive antibodies is regarded to be an important barrier limiting long-term outcome of allo- and xenografts. We asked whether enhanced signaling via the co-inhibitory receptor programmed cell death-1 (PD-1; CD279) can downregulate human B-cell activation.

METHODS

Proliferation of human purified CD19(+) B cells was induced by in vitro stimulation with CpG oligodeoxynucleotides (CpG-B). To induce antibody production, peripheral blood mononuclear cells were co-cultured with the porcine B-cell line L23. Triggering of inhibitory signals via the PD-1 receptor was obtained either using a recombinant agonistic soluble ligand (PD-L1.Ig) or L23 transfectants overexpressing membrane-bound human PD-L1 (CD274; L23-PD-L1 cells).

RESULTS

Stimulation of purified CD19(+) B cells with CpG-B resulted in upregulation of PD-1 and strong proliferation. Addition of PD-L1.Ig significantly reduced B-cell proliferation in a dose-dependent manner. A great proportion (~1%) of human circulating B cells recognizes the epitope galactose-α1,3-galactose-β1,4-N-acetylglucosamine-R (α-gal). Thus, when B cells-in the presence of T cell help-were cocultured with α-gal-expressing L23 cells, anti-gal and anti-L23 antibodies could readily be detected in the culture supernatant. The level of induced antibodies was significantly reduced when stimulation was performed by L23-PD-L1 cells.

CONCLUSIONS

Enhancing inhibitory signals may be part of future protocols to better control humoral immunity to allo- and xenografts.

siRNA mediated knockdown of tissue factor expression in pigs for xenotransplantation

Acute vascular rejection (AVR), in particular microvascular thrombosis, is an important barrier to successful pig-to-primate xenotransplantation. Here, we report the generation of pigs with decreased tissue factor (TF) levels induced by small interfering (si)RNA-mediated gene silencing. Porcine fibroblasts were transfected with TF-targeting small hairpin (sh)RNA and used for somatic cell nuclear transfer. Offspring were analyzed for siRNA, TF mRNA and TF protein level. Functionality of TF downregulation was investigated by a whole blood clotting test and a flow chamber assay. TF siRNA was expressed in all twelve liveborn piglets. TF mRNA expression was reduced by 94.1 ± 4.7% in TF knockdown (TFkd) fibroblasts compared to wild-type (WT). TF protein expression in PAEC stimulated with 50 ng/mL TNF-α was significantly lower in TFkd pigs (mean fluorescence intensity TFkd: 7136 ± 136 vs. WT: 13 038 ± 1672). TF downregulation significantly increased clotting time (TFkd: 73.3 ± 8.8 min, WT: 45.8 ± 7.7 min, p < 0.0001) and significantly decreased thrombus formation compared to WT (mean thrombus coverage per viewing field in %; WT: 23.5 ± 13.0, TFkd: 2.6 ± 3.7, p < 0.0001). Our data show that a functional knockdown of TF is compatible with normal development and survival of pigs. TF knockdown could be a valuable component in the generation of multi-transgenic pigs for xenotransplantation.

Human TNF-related apoptosis-inducing ligand-expressing dendritic cells from transgenic pigs attenuate human xenogeneic T cell responses

BACKGROUND

Efficient and precise techniques for the genetic modification of pigs facilitate the generation of tailored donor animals for xenotransplantation. Numerous transgenic pig lines exist with the focus on inhibition of the complement system and of humoral immune responses. In addition, immune cell-based responses need to be controlled to prevent pig-to-primate xenograft rejection. Expression of human (hu) TNF-related apoptosis-inducing ligand (TRAIL) on porcine cells has the potential to ameliorate human T cell responses.

METHODS

We generated transgenic pigs expressing human tumor necrosis factor (TNF)-related apoptosis-inducing ligand (huTRAIL) under the control of either the mouse H2K(b) promoter or a CMV enhancer/chicken β-actin (CAG) promoter, the latter one (CAG-huTRAIL) on a GGTA1 knockout/huCD46 transgenic background. The biological activity of huTRAIL was demonstrated by its apoptosis-inducing effect on Jurkat lymphoma cells. To clarify whether huTRAIL affects also primary immune cells and whether its effects depend on the presence of co-stimulatory molecules, we exposed human peripheral blood mononuclear cells (PBMC) or isolated T cells to huTRAIL-expressing porcine fibroblasts or dendritic cells in vitro.

RESULTS

H2Kb-huTRAIL transgenic pigs express huTRAIL mainly in the spleen and secondary lymphoid tissues. The CAG-huTRAIL construct facilitated huTRAIL expression in multiple organs, the level being at least one order of magnitude higher than in H2Kb-huTRAIL transgenic pigs. Incubation with huTRAIL-expressing H2Kb-huTRAIL transgenic porcine dendritic cells decreased human T cell proliferation significantly without any signs of apoptosis. In spite of the high transgene expression level, CAG-huTRAIL transgenic fibroblasts did not affect proliferation of human PBMC, independent of their activation state.

CONCLUSIONS

These results suggest huTRAIL expression on porcine dendritic cells as a possible strategy to attenuate T cell responses against pig-to-primate xenografts.

Transgenic expression of human heme oxygenase-1 in pigs confers resistance against xenograft rejection during ex vivo perfusion of porcine kidneys

BACKGROUND

The major immunological hurdle to successful porcine-to-human xenotransplantation is the acute vascular rejection (AVR), characterized by endothelial cell (EC) activation and perturbation of coagulation. Heme oxygenase-1 (HO-1) and its derivatives have anti-apoptotic, anti-inflammatory effects and protect against reactive oxygen species, rendering HO-1 a promising molecule to control AVR. Here, we report the production and characterization of pigs transgenic for human heme oxygenase-1 (hHO-1) and demonstrate significant protection in porcine kidneys against xenograft rejection in ex vivo perfusion with human blood and transgenic porcine aortic endothelial cells (PAEC) in a TNF-α-mediated apoptosis assay.

METHODS

Transgenic and non-transgenic PAEC were tested in a TNF-α-mediated apoptosis assay. Expression of adhesion molecules (ICAM-1, VCAM-1, and E-selectin) was measured by real-time PCR. hHO-1 transgenic porcine kidneys were perfused with pooled and diluted human AB blood in an ex vivo perfusion circuit. MHC class-II up-regulation after induction with IFN-γ was compared between wild-type and hHO-1 transgenic PAEC.

RESULTS

Cloned hHO-1 transgenic pigs expressed hHO-1 in heart, kidney, liver, and in cultured ECs and fibroblasts. hHO-1 transgenic PAEC were protected against TNF-α-mediated apoptosis. Real-time PCR revealed reduced expression of adhesion molecules like ICAM-1, VCAM-1, and E-selectin. These effects could be abrogated by the incubation of transgenic PAECs with the specific HO-1 inhibitor zinc protoporphorine IX (Zn(II)PPIX, 20 μm). IFN-γ induced up-regulation of MHC class-II molecules was significantly reduced in PAECs from hHO-1 transgenic pigs. hHO-1 transgenic porcine kidneys could successfully be perfused with diluted human AB-pooled blood for a maximum of 240 min (with and without C1 inh), while in wild-type kidneys, blood flow ceased after ∼60 min. Elevated levels of d-Dimer and TAT were detected, but no significant consumption of fibrinogen and antithrombin was determined. Microthrombi could not be detected histologically.

CONCLUSIONS

These results are encouraging and warrant further studies on the biological function of heme oxygenase-I expression in hHO-1 transgenic pigs in the context of xenotransplantation.

The 77C->G mutation in the human CD45 (PTPRC) gene leads to increased intensity of TCR signaling in T cell lines from healthy individuals and patients with multiple sclerosis

The 77C-->G mutation in exon A of the human CD45 gene occurs with low frequency in healthy individuals. An enhanced frequency of 77C-->G individuals has been reported in cohorts of patients suffering from multiple sclerosis, systemic sclerosis, autoimmune hepatitis, and HIV-1. To investigate the mechanisms by which the variant allele may contribute to disease susceptibility, we compared T cell reactivity in heterozygous carriers of the mutation (healthy individuals and multiple sclerosis patients) and wild-type controls. In vitro-generated T cell lines and freshly isolated CD4+CD45R0+ primed/memory T cells from 77C-->G individuals aberrantly expressed CD45RA isoforms and showed enhanced proliferation and IL-2 production when stimulated with anti-TCR/CD3 mAb or Ag. Mutant T cell lines contained a more active pool of p56lck tyrosine kinase and responded with increased phosphorylation of Zap70 and TCR-zeta and an enhanced Ca2+ flux to TCR/CD3 stimulation. These data suggest that 77C-->G may act as a risk factor for certain diseases by increasing the intensity of TCR signaling.

Modulation of human anti-pig T cell responses by monoclonal antibodies directed to porcine CD45 molecules

Antibody-mediated targeting of pig costimulatory molecules is assumed to be a possible strategy to achieve donor-specific tolerance after xenotransplantation. However, porcine molecules of the B7 family (e.g. CD86) are expressed on typical antigen presenting cells (APC) and also on vascular endothelial cells. Thus, in vascularized porcine xenografts the usage of therapeutic anti-B7 monoclonal antibodies (mAb) might be associated with damage of the endothelium.

OBJECTIVE

In the present study we asked whether modulation of human T cell reactivity can be obtained by targeting molecules selectively expressed on pig leucocytes.

METHODS

MAb directed to pig CD45 were tested for their capacity to modulate the in vitro activation of human T cells induced by porcine peripheral blood mononuclear cells.

RESULTS

Porcine stimulatory cells induced significant proliferation of human T cells. In the presence of porcine CD45 mAb human T cell responses were reduced by 30-40%. The inhibitory effects were most pronounced when CD45RA mAb were used whereas mAb directed to CD45RC isoforms only moderately inhibited human T cell activation. The tested antibodies had no effects on human T cell activation induced by mitogens or by alloantigen.

CONCLUSION

Manipulation of CD45 molecules on pig leucocytes may reduce their potential to stimulate human T cells. In recipients of vascularized porcine xenografts the usage of anti-pig CD45 mAb could be an approach to block the direct pathway of T cell activation initiated by porcine APC without affecting the endothelium of the graft.

Down-regulation of human alloimmune responses by genetically engineered expression of CD95 ligand on stimulatory and target cells

Transgenic expression of apoptosis-inducing molecules could be a strategy to protect cells and tissues from destruction by apoptosis-susceptible effector T cells. Some evidence for the potency of this approach has been obtained in mouse and rat transplantation models. However, limited data are available on the capacity of apoptosis-inducing molecules to modulate human alloimmune responses. In this study we analyzed the in vitro consequences of an interaction of human T cells with allogeneic 293 cells and 293 transfectants stably expressing high levels of the apoptosis-inducing CD95 ligand (CD95L). Both, CD95L(-) and CD95L(+) 293 cells were able to activate allogeneic T cells as demonstrated by comparable CD25 expression at day 2 of culture. The analysis of viable T cells at day 7, however, revealed anti-293 cytotoxic activity only in cultures that had been stimulated with CD95L(-) 293 cells. Alloactivated effector T cells lysed CD95L(-) and CD95L(+) 293 targets with similar efficiency when tested in a 4-h 51Cr-release assay. Prolongation of the effector phase to 20 h resulted in a further increase in the destruction of CD95L(-) target cells, whereas lysis of CD95L(+) targets remained low. These data suggest that genetically engineered expression of CD95L on cells or tissues could be an approach to control human T cell reactivity towards allografts. During the induction of an alloimmune response depletion of cytotoxic precursor cells may be obtained by overexpressing CD95L on stimulatory cells; CD95L expression on graft tissue might limit T cell-mediated destruction of the transplant during the effector phase of the response.